(a) Field of the Invention
The present invention relates to an optical signal processor used as a core component of a wavelength division multiplexing optical transmission and switching system. More specifically, the present invention relates to a high speed optical signal processor including a saturable absorber and a gain-clamped optical amplifier, which has satisfactory characteristics even when an input optical signal is a high rate pulse or a pulse having a narrow width.
(b) Description of the Related Art
A saturable absorber has been frequently used for a pulse laser such as a mode-locking laser diode, and recently it has been applied to optical signal processing devices for noise reduction and light detection.
Luminous intensity variation and absorption coefficient characteristics obtained in the case that a conventional saturable absorber is used are shown in FIG. 3. That is, input light having power of less than a transparent input power is absorbed, but most input light with power of more than the transparent input power is transmitted with only a little loss generated. Accordingly, in the case that a signal light having noise is injected into the saturable absorber, the noise is absorbed by the saturable absorber and removed when it has intensity lower than the transparent input power.
Furthermore, the relation between time and a variation in absorption coefficient of an input signal pulse having a narrow width of several ps is shown in FIG. 4. In the graph of FIG. 4, the absorption coefficient at the rising leading edge where luminance intensity of the optical signal pulse starts to increase is dropped at a rapid response speed. However, the absorption coefficient is recovered very slowly from the dropping trailing edge at which luminance starts to decrease. This absorption coefficient variation is caused by a long lifetime of a carrier. Consequently, when light inputted into the saturable absorber includes a pulse having a width narrower than the carrier lifetime, the saturable absorber cannot fulfil its noise removal function.
A semiconductor optical amplifier conventionally used in an optical communication system creates an amplified spontaneous emission during its amplification process. At this time, a noise in the form of a pulse with a narrow width cannot be removed with the saturable absorber according to the above-described reason, so that it largely restricts a transmission distance.
Accordingly, it is possible to evade the restriction in data transmission band width and transmission distance only when the carrier lifetime is reduced such that the saturable absorber can absorb even the pulse with a narrow width to remove it. In this case, the absorption coefficient with respect to time when an input pulse having a width of several ps is injected into the saturable absorber is dropped at a fast response speed at the rising leading edge where luminance intensity of the optical signal pulse starts to increase, as shown in FIG. 5. Furthermore, the absorption coefficient is recovered symmetrically with the luminance intensity variation even at the dropping trailing edge at which the luminance intensity starts to decrease. This means that stabilized characteristics can be obtained in the case that the carrier lifetime is reduced.
Methods for reducing the carrier lifetime include a method of injecting a heavy ion into the saturable absorber to generate defects therein, as described in an article entitled “Topics in Quantum Electron”, in IEEE, No. 2, Vol. 7, March 2002; a method of applying a reverse voltage to the saturable absorber to expel carriers generated due to light absorption in the saturable absorber from the absorber, as described in J. Lightwave Technol, Vol. 10, 1992; a method of growing a saturable absorption layer at a temperature lower than the conventional case to form a vacancy and defect, as described in IEE Electron, Lett Vol. 3, 1995; and a method of continuously inputting light to the saturable absorber to remove the excited carriers in the saturable absorber by stimulated emission, as disclosed in U.S. Pat. No. 5,805,327.
However, the aforementioned methods require a separate ion implanting apparatus, cause deterioration in the quality of an epitaxial layer due to low-temperature growth, or need additional apparatuses such as a light-emitting device and a voltage applying device.